Several cameras and objectives are necessary for taking shots of images from several different angles of view. Stereoscopic rendition may rely solely on two images, a left image and a right image or on a set of several images only one pair of which is visible to the viewer as a function of his position in front of the projection device. The latter system allows the dissemination of so-called “multiview” 3D images. The viewer moving in front of the 3D projected image sees the objects represented move as they would do in reality.
“3D” vision corresponds to an interpretation by the brain of images perceived by the two eyes so as to give a position to objects in space. The position ascribed depends on the differences between the images perceived by the viewer's two eyes. If the object is in motion, the images must be synchronous, taken at the same instant, so that the displacement of the object between two successively taken images, left and right, is not interpreted as distance information.
Stereoscopic optical solutions using just a single sensor are known from the prior art. Notably, patents DE 10 2004/052253 and EP 0 969 308 which describe stereoscopic devices comprising two input optics and a single sensor to reproduce two viewpoints will be cited. Solutions using just a single head optic are also known. Patents U.S. Pat. No. 6,335,833 and JP 2008/292513 will be cited in this category. However, most picture taking systems comprise a pair of, generally identical, channels.
In a stereoscopic system of this type, each channel or camera comprises three main sub-assemblies, namely:
A sensor casing comprising a photosensitive sensor and the associated electronic control means;
An optical objective which is generally a zoom whose field is variable. On this type of optic, three parameters at least are monitored, the focal length which determines the field, the focusing which determines the sharpness distance and the aperture which determines the illumination received by the detector;
An electromechanical device making it possible to control the values of the various parameters.
The two cameras are then mounted on a common mechanical platform which comprises inter-axis spacing and convergence adjustments. In a first embodiment of the picture taking system, the cameras are simply placed side by side, the optical axes of the objectives being in one and the same plane, the optics being able to converge slightly. Their inter-axis spacing, ever in the same plane, is also adjustable. The interpupillary distance makes it possible to tailor the degree of stereoscopy. The drawback of this embodiment is that the distance separating the two optical axes of the objectives necessarily has a minimum value related to the bulkiness of the optics and of their supports. In certain picture taking configurations, the stereoscopic effect is thus limited.
The system S3D represented in FIG. 1 does not have this drawback. It comprises two cameras C1 and C2 disposed perpendicularly with respect to one another and separated by a semi-reflecting plate L disposed at 45 degrees to the axes of the objectives of the cameras. In this configuration, the mechanical constraint disappears and it is possible to separate the optical axes by the desired distance at the price, however, of a photometric attenuation and of a slight optical path difference on one of the pathways due to the thickness of the semi-reflecting plate.
It is notably possible to reproduce zero inter-pupillary distances. The inter-axis spacing and the vergence between the cameras/objectives remain adjustable. This mechanical arrangement is unfortunately fairly bulky and the opto-mechanical assembly can be tricky to manipulate, particularly for portable systems.